The Mechanical Reliability of Sputter-Coated Indium Tin Oxide Polyester Substrates for Flexible Display and Touchscreen
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The Mechanical Reliability of Sputter-Coated Indium Tin Oxide Polyester Substrates for Flexible Display and Touchscreen Applications Darran R. Cairns, David C. Paine and Gregory P. Crawford Brown University, Division of Engineering, Providence, RI 02912, U.S.A ABSTRACT Indium tin oxide (ITO) films deposited on polyester substrates are a key material in the development of two exciting technologies, touchscreens and flexible liquid crystal displays. The new generation ‘plastic’ displays and touchscreens must be flexible and robust, have excellent optical properties, and be inexpensive. We report on the mechanical and electrical reliability of ITO on a polyethylene terephthalate substrate (PET). We show that the mechanical behavior of the ITO film is dominated by the properties of the substrate and that the deformation of the substrate is mapped by the crack patterns in the ITO. This is most strongly evidenced in the simulated wear of a touchscreen where failure after >50000 pen strokes is primarily due to cracking of the ITO as a result of increased substrate deformation over time. In addition the mechanical reliability of the ITO layer is dependent on the film thickness. Cracking was observed in a 105 nm thick ITO film at a strain of 0.022 and for a 16.8 nm thick film at 0.003. The thickness and hence sheet resistance of the film effectively limits the maximum allowable deformation of the substrate and must be considered in the design of suitable display and touchscreen devices. In addition we report on the change in resistance with time-attemperature and relate this to the shrinkage of the substrate.
INTRODUCTION Almost everywhere we look there are touch-input devices ranging from personal digital assistants (PDAs) to automatic telling machines (ATMs) and point of sale (POS) cash registers. There is also an increasing number of new touch enabled internet devices and even voting machines. One unifying feature of many of these devices is the use of transparent conducting oxide (TCO) coatings as an integral component of the touch device. Unlike many applications of TCOs touchscreens – as the name suggests – are designed to be touched. This raises a number of issues concerning the mechanical reliability of the TCO layer including wear resistance, resistance to indentation and adhesion of the TCO to the substrate. In addition to touchscreens, which are already present in everyday life, there is an increasing interest in the possibility of flexible displays. Glass coated with a TCO layer, predominantly indium tin oxide (ITO), has been used for a number of years in the flat panel display industry. Polymers coated with ITO are now being considered for a number of display applications where the light weight, flexibility and toughness of F3.24.1
polymers compared to glass open up new applications. For flexible display applications the TCO layer may be subjected to deformation and the mechanical reliability of the ITO layer must be understood. Touchscreens There are a number of touchscreen technologies but the two most wide
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